Catalysts containing crystalline zeolites dispersed in an inorganic oxide matrix have been used for the catalytic cracking of petroleum-derived feedstocks for many years. During this time, it has been widely recognised in the industry that certain contaminants (notably vanadium, nickel, and iron), initially dissolved or dispersed in the hydrocarbon feedstock, are deposited on the catalyst during the catalytic cracking process, and the accumulated deposits lead to undesirable changes in the activity and selectivity of the thus contaminated catalysts. Typically, the harmful effects noted have been increased yields of coke and hydrogen, a phenomenon ascribed to the action of the deposited metals as centres of dehydrogenation. More recently, however, it has been appreciated that vanadium also has other harmful properties, as well as increasing dehydrogenation activity, it reacts with and destroys the zeolite component of the catalyst, leading to a severe decrease in the activity of the catalyst.
These problems have become more acute as refiners have faced the need to process heavier feedstocks which contain increased amounts of the metal contaminants, and various strategies have been employed to alleviate the deleterious effects and facilitate smooth running of catalytic cracking units. These approaches have included.
(1) more frequent replenishment of the circulating catalyst inventory;
(2) withdrawal of the regenerated catalyst and treatment with various chemicals to passivate the metals;
(3) changes in the design or operation of the catalytic cracker to reduce the poisoning activity of the contaminant metals;
(4) addition to the feedstock of compounds of elements such as antimony, tin, barium, manganese, germanium and bismuth.
Examples of these approaches will be found in the following patents: U.S. Pat. Nos. 4,111,845, 4,101,417, 4,377,494, 4,367,136, 3,977,963.
Further attempts to cope with harmful effects of metals, especially vanadium, have related to modifications of the cracking catalyst itself; these have included admixture with sacrificial catalyst particles, careful control of the zeolite composition, and inclusion in the catalyst of specified amounts of vanadium trapping additives, including alumina, titanium dioxide (titania) and zirconium dioxide (zirconia) and certain compounds of calcium and magnesium. Disclosures of such catalysts will be found in U.S. Pat. Nos. 4,432,890, 4,451,355 and BE 899,446.